1. Field of the Invention
The present invention relates to methods for synthesizing zeolites from Y-zeolite. More particularly, the present invention relates to methods for synthesizing synthetic analogues of heulandite, brewsterite, epistilbite, harmotome and gmelinite zeolites containing alkaline earth cations using Y-zeolite as a starting material
2. Description of the Related Art
Ever since Barrer's pioneering work (Barrer, et al., J. Chem. Soc., (1961), 971), numerous studies in zeolite synthesis have concentrated on the use of organic structure-directing agents (SDAs) in the synthesis mixture that affect the crystallization of zeolite. Debate continues on the roles of various types of organic components in the crystallization process.
One problem with using organic reagents in zeolite synthesis is their cost. In order to reduce the expense of zeolite production, syntheses that do not use SDAs are desirable. Many natural zeolites that have unique structures have not been synthesized. Thus, new routes to synthesize known active zeolites without organic reagents could provide low cost materials Syntheses of the heulandite (HEU) family of zeolites, the most abundant zeolites found in nature, through a number of routes have been reported by Satokawa, et al. Microporous Mater., 8 (1997), 49; Williams, Chem. Commun., (1997), 2113; Zhao, et al., Zeolites, 19 (1997), 366-369; and Zhao, et al., Microporous Mater., 21 (1998), 371-379. These syntheses involve conventional hydrothermal crystallization using alkaline metal cations and amorphous oxides (not zeolites).
Most natural zeolites have alkaline earth cations as the dominant cation (i.e., highest concentration relative to all other metal cations present) in their composition. Because of this fact, we studied use of alkaline earth cations to prepare HEU, harmotome (PHI), brewsterite (BRE), epistilbite (EPI) and Yugawaralite (YUG) type zeolites from either zeolite P or L, as reported in Khodabandeh, et al., Microporous Mater. 12:(4-6) 347-359 (December 1997) Khodabandeh, et al., Microporous Mater., 11:(1-2) 87-95 (August 1997), Khodabandeh, et al., Microporous Mater., 9: (3-4) 149-160 (September 1997), Khodabandeh, et al., Chem. Comm., (10) 1205-1206 (May 1996). Of importance to our synthetic methodology is the starting material, e.g., the ratio of Si/Al and framework density. Hydrothermal conversions without organic SDAs can be effected only by converting a zeolite with a relatively lower framework density to one of a relatively higher framework density. Table 1 lists framework density for several zeolites.
TABLE 1 Exemplary Framework Densities Zeolite Framework Density FAU (Y-zeolite) 12.7 GME 14.6 *BEA (zeolite beta) 15.0 GIS (P-zeolite) 15.4 BOG 15.6 PHI 15.8 LTL (L-Zeolite) 16.4 HEU 17.0 BRE 17.5 EPI 18.0 YUG 18.3
The choice of the starting material for zeolite synthesis with alkaline earth cations is of critical importance.
In 1960, Koizumi and Roy reported synthesis of a heulandite-type zeolite from the composition CaO.Al.sub.2 O.sub.3 .7SiO.sub.2.5H.sub.2 O at temperatures between 250.degree. C. and 360.degree. C. and a pressure range of 15,000 to 37,000 psi. In 1981, Wirsching obtained heulandite by hydrothermal alteration of rhyolitic glass under the action of CaCl.sub.2 solutions at temperatures of 200.degree. C. to 250.degree. C. and reaction times of around 80 days. Additionally, some syntheses for clinoptilolite zeolites have been reported.
Without methods described herein or others developed by one of the inventors, it is either difficult or impossible to produce other zeolites synthetically. For example, gmelinite cannot be synthesized from zeolite P or L because the framework density gmelinite is relatively lower than that of either zeolite P or L.
Harmotome, another rare zeolite of hydrothermal origin which has the phillipsite (PHI) topology, is characterized by three dimensional channels consisting of pores composed of eight tetrahedral atoms. The dominant cation in the zeolite is Ba.
Epistilbite, another rare zeolite of hydrothermal origin, has previously been produced by hydrothermal treatment of rhyolytic glass at 250.degree. C. and from powdered SiO.sub.2 glass at 250.degree. C. It has a structure characterized by intersecting channels composed of eight and ten tetrahedral atoms.
Zeolite beta (*BEA) (occurs naturally as mineral Tschernichite) and Boggsite (BOG) each can be synthesized from Y zeolite according to the methods of the present invention. Tschernichite has a Si/Al of about 5, but this ratio has not been synthesized Boggsite has Si/Al of about 5, but as far as the inventors are aware, has not yet been synthesized.
Since the naturally occurring materials are rare, but so potentially useful, it would be advantageous to enable less rigorous and therefore less expensive routes for producing greater quantities of these materials. Particularly of interest are routes that can produce materials with few or no impurities, i.e., anything other than the desired zeolite product (e.g., other zeolite by-product).
Y-zeolite (FAU) has long been used as an industrial catalyst due to its high activity and low cost. Furthermore, Y-zeolite can be used as an aluminum source for ZSM-5 synthesis (e.g., as reported by Bourgogne, et al., U.S. Pat. No. 4,503,024) and to prepare novel materials such as zeolite beta with low Si/Al ratios (Zones, et al., U.S. Pat. No. 5,340,563). When Y-zeolice can be used as an aluminum source in zeolite synthesis, the type of cations for ion exchange is very important. For example, although relative to the control (Na--Y), Co--Y or Cu--Y reaction rates to chabazite were just as fast, Fe and Cr inhibit this reaction completely, as reported by Zones, J. Chem. Soc. Farad. Trans., 86 (1990), 3467 and 87 (1991), 3709 and Stud. Sur. Sci. Catal. Vol. 97, pp. 45-52 (1995).
Here, we focus on the use of Y-zeolite as a starting material for hydrothermal conversion with alkaline earth cations. Exemplary conversions of Y-zeolite to analcime (ANA) and synthetic analogues of heulandite, gmelinite (GME), harmotome and brewsterite zeolites are presented. The factors that determine the products are shown to be the Si/Al ratio in the starting zeolite, the presence or absence of seeds, the composition of the reaction medium, and the reaction time.
As one of ordinary skill would recognize, syntheses of other zeolites from Y-zeolite of relatively higher Si/Al, i.e., greater than 3.5, can utilize a commercially prepared Y-zeolite or can be synthesized, as is taught in the literature.